Exhaust gas recirculation (EGR) systems are being increasingly utilized to improve engine efficiency and reduce the harmful effects of exhaust emissions on the environment. As an engine burns fuel, it produces an exhaust gas containing unburned fuel, also known as hydrocarbons (HC), carbon monoxide (CO), and nitrous oxides (NOx). The exhaust gas is redirected through the engine to consume the unburned fuel and results in a dilution of the combustion mixture. This dilution results in a reduction of the percentage of fresh intake air in the combustion mixture and leads to reduced formation of CO and NOx. Burning the exhaust gas before it is released reduces the harmful effects of the exhaust gas on the atmosphere and enables the vehicle to meet government emission standards.
EGR systems typically include an EGR cooler that cools the EGR as it flows back to the engine intake. Cooled external EGR is proposed to improve fuel economy on gasoline engines via reducing throttling losses, reducing in-cylinder heat losses, mitigating knock, and decreasing enrichment requirements. As EGR passes through the EGR cooler, fouling of the cooler may occur via deposition of particulate matter in the cooler. For typical EGR systems, EGR coolers require replacement after a certain amount of fouling. Fouling of the EGR cooler reduces the cooling effectiveness of the EGR cooler, increases its pressure drop, and reduces the ability of the cooled EGR to increase fuel economy by minimizing its in-cylinder benefits and flow capacity.
Further, cooled EGR systems reach limits, reducing the benefits of cooled EGR. In high load EGR systems, such as gasoline turbocharged direct injection (GTDI) low pressure (LP) EGR, coolant flow or radiator heat rejection limitations can result in reduced EGR rates or even result in inability to flow any EGR. In transient cases, external EGR is frequently turned off to help improve torque response of the engine including maximizing turbo response.
Another issue with EGR systems is an inability to meet engine dilution requirements. Modern methods have been introduced to supplement EGR systems and meet engine dilution demands. One example approach is shown by Liederman et al. in U.S. Pat. No. 6,668,766. Therein, a direct injector is coupled to an engine cylinder and the injector is activated to inject water to the cylinder when EGR is unable to meet dilution requirements.
However, the inventors have recognized potential issues with such systems. As one example, direct injection of water at an engine cylinder does not allow the system to clean the EGR cooler when a particulate matter load is greater than a threshold particulate load. As a result, if the EGR cooler had a high amount of particulate matter, then EGR flow to the engine would be compromised. As a result, a standard cooler regeneration would be carried out, which typically reduces fuel economy. This may lead to unstable combustion and degrading engine performance and emissions.
Accordingly, methods and systems are provided herein in order to at least partly address the above issues. In one example, an engine method comprises, in response to a dilution requirement, direct injecting fluid at an EGR cooler. A rate of the direct injecting may be based on each of an EGR flow rate and an estimate of compressor blade speed.
Herein, the inventors have found direct injecting water at an EGR cooler can help meet engine dilution demands and/or clean the EGR cooler. For example, the EGR cooler direct injection may be halted or reduced during conditions of high compressor blade speeds in order to reduce damage to the blades that may occur if the injection were to dislodge any particulate matter stored in the cooler. Further, the direct injector may be used during instances of limited EGR flow to meet engine dilution demands, and/or enable increased EGR by limiting over-temperature issues in the compressor and/or EGR cooler. Direct injection of water into the EGR cooler to meet dilution demands may improve engine performance and efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.